It was quite difficult in the prior art to firmly join silicone rubber to polyolefin resins such as polyethylene, polypropylene and fluorine resins. Silicone rubber could be attached to nylon resins and polycarbonate resins by applying a primer prior to the silicone rubber or by moderately heat curing an uncured silicone rubber over a relatively long time, but the bonding force was low.
Silicone rubbers found wide-spread use in electric, automobile and other applications since they were recognized to be reliable with respect to heat resistance, weather resistance and electrical properties. However, therefor was a need for a method capable of firmly joining silicone rubber to thermoplastic resin to form an integrated article.
Several proposals were made to meet such a need. Japanese Patent Application Kokai (JP-A) No. 178048/1985 discloses to add an organohydrogenpolysiloxane containing more than 30 mol % of an organohydrogensiloxane unit to an olefin resin in order to bond silicone rubber to the olefin resin. Since this organohydrogenpolysiloxane has a relatively long polysiloxane chain and is less compatible with a thermoplastic resin, it can give rise to undesired phenomena like separation and largely affect the mechanical properties of molded thermoplastic resin, especially resulting in a loss of modulus. Also JP-A 183843/1988 discloses that silicone rubber can be bonded to an olefin resin by adding to the olefin resin a silicon compound containing an organic group having at least one aliphatic unsaturated bond and at least one hydrolyzable group attached to a silicon atom in a molecule. This silicon compound is free of the above-mentioned drawbacks of JP-A 178048/1985, but is less reactive with silicone rubbers, especially addition type silicone rubbers so that no satisfactory adhesion is obtained unless a primer is used. There is commercially available a silane-crosslinked polyethylene which falls within this concept. Our attempt to cure addition type silicone rubber directly to the silane-crosslinked polyethylene failed to achieve a satisfactory bond.